Crossed-field microwave tubes having an improved control electrode geometry

ABSTRACT

An improved control electrode geometry for crossed-field microwave tubes is disclosed. The control electrode geometry is disposed facing the microwave slow wave circuit and includes an upstream portion which is recessed from the anode microwave circuit by more than the downstream portion of the control electrode. With a certain positive bias potential applied to the control electrode relative to the cathode potential a certain fraction of the electron stream will be collected on the control electrode in the absence of microwave energy on the microwave circuit, but in the presence of microwave energy on the microwave circuit the microwave energy interacts with the electrons to decrease the fraction of electrons collected on the control electrode.

atent [191 Wilczek [451 May 15, 1973 [54] CROSSED-FIELD MICROWAVE TUBES HAVING AN IMPROVED CONTROL ELECTRODE GEOMETRY [52] US. Cl. ..3l5/3.6, 315/39.63, 330/42 51 Int. Cl. ..H0lj 25/34 [58] Field of Search ..3l5/3.-5, 3.6, 5.11,

[56] References Cited UNITED STATES PATENTS 3/1970 Farney ..3l5/5.ll 11/1971 Dudley ..315/5.ll

Primary ExaminerCarl D. Quarforth Assistant Examirter-J. M. Potenza Attorney- Leon F. Herbert [57] ABSTRACT An improved control electrode geometry for crossedfield microwave tubes is disclosed. The control electrode geometry is disposed facing the microwave slow wave circuit and includes an upstream portion which is recessed from the anode microwave circuit by more than the downstream portion of the control electrode. With a certain positive bias potential applied to the control electrode relative to the cathode potential a certain fraction of the electron stream will be collected on the control electrode in the absence of microwave energy on the microwave circuit, but in the presence of microwave energy on the microwave circuit the microwave energy interacts with the electrons to decrease the fraction of electrons collected on the control electrode.

6 Claims, 3 Drawing Figures PATENTEDHAYI 51m 3. 733 509 FIG.|

M OUT IIIIIII IN 8 \D ,II III/ INVENTOR.

ANDREW S. WILCZEK BY ATTORNEY CROSSED-FIELD MICROWAVE TUBES HAVING AN IMPROVED CONTROL ELECTRODE GEOMETRY DESCRIPTION OF THE PRIOR ART Heretofore, it has been proposed, in US. Pat. application Ser. No. 668,784, filed Sept. 19, 1967 and assigned to the same assignee as the present invention, to provide a crossed-field amplifier with a control electrode structure biased with a static positive potential with respect to the cathode and disposed opposite the microwave circuit. The static bias potential supplied to the control electrode structure is selected, relative to the anode-to-cathode potential, to collect just a sufficient percentage of the electron stream, in the absence of r.f. energy on the circuit, to terminate operation of the tube. However, in the presence of a strong radio frequency field on the microwave circuit the field interacts with the secondary electron stream to replenish the stream by back-bombardment of the secondary emissive cathode sole to sustain a recirculating electron stream in the interaction region and to maintain operation of the tube. The statically biased control electrode, in such a crossed-field amplifier tube, greatly reduces the complexity of the tube and of the control electrode modulator. When a crossed-field tube is employed as a pulse amplifier in high pulse repetition rate radar systems, i.e., pulse repetition rates on the order of 1 MHz, the pulse modulator required to modulate the control electrode at these repetition frequencies becomes very complex and bulky. Thus, the provision of the statically biased control electrode represents a substantial improvement. However, it is desired to improve the performance of the statically biased control electrode.

SUMMARY OF THE PRESENT INVENTION The principal object of the present invention is the provision of an improved control electrode geometry for crossed-field microwave tubes.

One feature of the present invention is the provision of a control electrode geometry wherein the control electrode is recessed from the anode microwave circuit at the upstream end of the control electrode to provide a greater anode-to-cathode electrode spacing at the upstream end of the control electrode than at the downstream end thereof such that the control function of the control electrode is more sensitive to the presence of microwave energy on the microwave circuit.

Another feature of the present invention is the same as the preceding feature wherein the control electrode includes an abrupt projection at the downstream end thereof which projects toward the anode microwave circuit, such projection serving, in the absence of microwave energy on the microwave circuit, to enhance collection of electrons from the circulating electron stream and in the presence of microwave energy on the microwave circuit to move the electron stream closer to the anode circuit for enhanced interaction therewith to prevent collection of the electrons by the control electrode structure.

Another feature of the present invention is the same as any one or more of the preceding features wherein the control electrode structure is disposed in a microwave tube of circular geometry having a re-entrant electron stream and a field-free drift region with the cathode in the drift region being contoured to provide an increased anode-to-cathode spacing at the downstream end thereof, such spacing being approximately equal to the anode-to-control electrode spacing at the recessed upstream end of the control electrode.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a transverse schematic line diagram of a crossed-field microwave tube employing features of the present invention,

FIG. 2 is a schematic linearized line diagram depicting the crossed-field interaction region and the control and drift space cathode geometry of the present invention, and

FIG. 3 is an enlarged sectional view of a portion of the structure of FIG. 1 taken along line 3-3 in the direction of the arrows.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1 there is shown a crossedfield microwave tube 1 incorporating features of the present invention. The tube 1 includes a cathode electrode structure 2 concentrically surrounded by an anode electrode structure 3. A crossed field interaction region 4 is defined in the annular unobstructed space between the cathode and anode structures 2 and 3, respectively. A magnet, not shown, produces an axially directed magnetic field H in the annular crossed-field interaction region 4.

The anode structure 3 includes a microwave slow wave circuit portion 5, such as a double helix coupled bar circuit, which faces the cathode electrode 2. A conductive circuit sever 6 interrupts the microwave circuit 5 to define an upstream end 7 and a downstream end 8 to the circuit 5 adjacent the circumferential ends of the circuit sever 6. The circuit sever 6 also defines a microwave field-free drift region 9 in the space between the cathode electrode 2 and the circuit sever 6 for debunching of the spokes of electron space charge which circulate around the interaction region 4 in a re-entrant stream manner.

The cathode electrode structure 2 includes a control electrode structure 11 supported in insulative relation to the remaining portion of the cathode electrode 12 such remaining portion forming a cold cathode secondary emitter 12. The control electrode 11 is disposed facing the upstream end of the microwave circuit 5 and is supplied with an operating potential from source 13. The operating potential is a relatively static d.c. potential which is approximately one-half of the potential applied between the anode 3 and cathode 2 via power supply 14. In a typical example, the anode voltage V is approximately 30 kv more positive than the potential applied to the secondary emitter portion 12 of the cathode electrode structure 2. The static potential applied to the control electrode 11 is, thus, approximately 15 kv positive with respect to the potential applied to the secondary emitter portion 12 of the cathode 2.

Input microwave energy to be amplified is applied to the upstream end 7 of the anode microwave circuit 5 via input coaxial line 15. The microwave energy on the circuit cumulatively interacts with the electrons in the annular crossed-field interaction region 4 between the anode and cathode to produce an amplified microwave output signal at the downstream end 8 of the microwave circuit 5. The amplified output signal is extracted from the circuit 5 via an output coaxial line 16 and transmitted to a suitable utilization device, such as an antenna, not shown.

At the output end 8 of the microwave circuit 5, the electrons in the electron stream are formed into spokes of rotating space charge circulating around the cathode electrode structure 2. These spokes of space charge drift into the drift region 9 wherein they tend to debunch or to break apart due to space charge repulsion forces. In addition, the debunching action is enhanced by the contoured surface of the cathode electrode 2 which faces the circuit sever 6. More particularly, this portion of the cathode 2 is circumferentially contoured to provide an increased anode-to-cathode spacing at the downstream end of the drift region 9 relative to the anode-to-cathode spacing at the upstream end of the drift space 9. The increased spacing produces a reduction in the electric field between the anode and cathode and causes the spokes of electrons to slow down in their circumferential velocity and to generally disperse. In addition, this portion of the cathode surface is contoured in the axial direction, as shown in FIG. 3, by a plurality of steps 18. The steps 18 are circumferentially directed and, in addition, are arranged such that the cathode-to-anode spacing at the axial center plane 19 is substantially increased as compared to the spacing between the anode and cathode at the axial ends of the interaction region 4. This causes the electrons in the center, near the center plane 19, to take on a substantially slower circumferential velocity than those near the axial ends of the interaction region 4, thus, producing a shearing action and a further debunching action within the electron stream. In addition, the increased spacing between the cathode and anode near the center plane 19 causes the electron stream to be concentrated near the center plane 19 such that the electron stream is moved into the region of most intense electric field of the microwave circuit 5 as the debunched beam enters the interaction region adjacent the upstream end of the microwave circuit 5. The increased spacing between the circuit sever 6 and the cathode 2, at the downstream end of the drift region 9, causes the electron stream to move away from the anode circuit at the upstream end thereof. This is best seen in FIG. 2. The contoured cathode to define the aforedescribed drift space geometry is disclosed and claimed in copending U.S. patent application Ser. No. 829,640, filed June 2, 1969, now U.S. Pat. No. 3,614,515, issued Oct. 19, 1971 and assigned to the same assignee as the present invention.

The positive static bias applied to the control elec- 'trode 11 causes a certain fraction of the electrons within the electron stream to be collected on the control electrode 11. It further causes the electron stream to be deflected away from the anode circuit and toward the biased control electrode 11. The bias potential applied to the control electrode 11 is adjusted such that the biased electrode collects just a sufficient percentage of the electron stream such that the uncollected portion of the electron stream which passes the bias electrode without being collected thereon, in the absence of applied microwave energy on the slow wave circuit 5, is insufficient to sustain operation of the tube and the tube is thus turned off.

However, in the presence of microwave energy on the circuit 5 a sufficient number of the electrons are pulled by the microwave fields passed the biased electrode structure such that a certain fraction of the electrons are driven back into the cathode surface by the microwave energy to produce back-bombardment and secondary emission to sustain the electron stream.

The biased control electrode 1 1 has a geometric configuration which enhances its sensitivity to the presence of microwave energy on the slow wave circuit. More particularly, the upstream end of the bias electrode 11 is recessed away from the anode circuit 5 such that the spacing between the anode circuit and the bias control electrode 11 at the upstream end of the electrode is substantially greater than the spacing between the bias control electrode 11 and the anode circuit at the downstream end thereof. The downstrearn end of the electrode preferably includes an abrupt projection 19 projecting toward the anode circuit 5. This projection 19 serves two functions. First, it tends to increase the electron collecting function of the bias control electrode in the absence of microwave energy on the slow wave circuit 5 since the electrons which have been deflected toward the biased control electrode, but which have not been collected in the recessed portion thereof, have sufficient inertia such that they cannot be deflected toward the anode sufficiently abruptly to prevent being intercepted by the projection 19. Thus these electrons are collected and removed from the electron stream.

However, in the presence of microwave energy on the microwave circuit 5, the microwave fields interact on the electrons tending to slow these electrons down and to pull them toward the anode circuit 5 and around the projection 19, as indicated by the dotted lines in FIG. 2. Thus, those electrons of the electron stream which have the proper phase to deliver energy to the slow wave circuit 5 are deflected by the r.f. energy around the projection 19 to contribute to the electron stream to produce a cumulative interaction which produces further back-bombardment of the cathode, downstream of the control electrode, to renew the electron stream and to produce further cumulative interaction and amplification of wave energy on the circuit. Thus, the projection 19 as combined with the recessed portion of the biased control electrode 11 serves to increase the sensitivity of the control function of the control electrode 11 to the presence or absence of microwave energy on the slow wave circuit. Thus, the efficiency of the control electrode 11 is substantially improved as compared to prior art control electrodes which were merely arcuate sectors flush with the adjacent cathode sections.

In a typical example of the control electrode 11, the control electrode occupies approximately 50 of arc which corresponds to approximately 20 percent of the active microwave circuit length. In addition, the upstream end of the control electrode is recessed such that the spacing from the anode circuit to the control electrode at the upstream end of the electrode is at least 1% times the spacing from the anode circuit to the downstream projection 19. Moreover, additional control electrode structures 11 may be employed around the periphery of the cathode structure 2 with intervening secondary emitter sections 12.

If the anode microwave circuit 5 is relatively long, a plurality of such control electrodes 11 are desirable since noise energy in the electron stream can produce a wave on the circuit which cumulatively interacts with electrons in the interaction region 4 to produce backbombardment of the cathode to start operation of the tube in the absence of an applied microwave signal on the circuit. Moreover, the plurality of such control electrodes 11 reduces the amount of circulating current that must be collected by each of the electrodes because the electron stream does not have an opportunity to build up such a great intensity before it encounters the next control electrode.

Although the improved control electrode geometry of the present invention has been shown in FIG. 1 as it is employed in a crossed-field tube of circular format with the anode surrounding the cathode, this is not a requirement and the electrode geometry may be equally well employed in tubes wherein the cathode surrounds the anode circuit. Moreover, the control electrode geometry 11 is not restricted to use in crossed-field tubes of circular format but may be utilized to advantage in crossed-field tubes of a linear format where the electron stream is non-reentrant.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a crossed-field microwave tube, means forming a cathode electrode structure having a secondary cathode emitter portion for generating a stream of secondary electrons by electron back-bombardment of said secondary emitter portion, means forming an anode electrode structure having a microwave slow wave circuit portion spaced from and facing said cathode electrode structure for electromagnetic interaction with the stream of secondary electrons to define a crossed-field interaction region in the space between said cathode and anode, means for applying microwave energy to be amplified to said anode circuit, said cathode electrode structure including a control electrode portion facing said microwave circuit for terminating amplification of the input energy upon termination of the input microwave energy as applied to said microwave circuit, means for insulating said control electrode portion from said secondary emitter portion of said cathode structure, the improvement wherein, the surface of said control electrode which faces said anode circuit is recessed from said anode circuit at the upstream end thereof relative to the downstream end thereof to provide greater anode-to-control electrode spacing at the upstream end of said control electrode than at the downstream end of said control electrode.

2. The apparatus of claim 1 wherein said control electrode includes an abrupt projection at the downstream end thereof, such projection projecting toward said anode circuit.

3. The apparatus of claim 1 wherein the anode-tocontrol electrode spacing is at least 1% times as great at the upstream end of said control electrode as at the downstream end of said control electrode.

4. The apparatus of claim 1 including means for applying a bias potential to said control electrode portion which is positive relative to the potential applied to said secondary emitter portion of said cathode structure.

5. The apparatus of claim 1 wherein said anode and cathode electrode structures are concentrically disposed to define an annular unobstructed reentrant stream crossed-field interaction region therebetween, and said anode including a circuit sever portion interrupting said microwave circuit portion to define an input and an output end of said anode circuit adjacent the ends of said circuit sever.

6. The apparatus of claim 5 wherein said circuit sever defines a microwave field-free drift region in the space between said sever and said cathode electrode, the surface of said cathode being contoured such that the spacing between said cathode and said circuit sever is greater at the downstream end of said drift region than at the upstream end thereof, and wherein the spacing between the recessed upstream end of said control electrode and said microwave circuit is approximately equal to the cathode-to-anode spacing at the downstream end of said drift region on the axial center plane of the interaction region. 

1. In a crossed-field microwave tube, means forming a cathode electrode structure having a secondary cathode emitter portion for generating a stream of secondary electrons by electron backbombardment of said secondary emitter portion, means forming an anode electrode structure having a microwave slow wave circuit portion spaced from and facing said cathode electrode structure for electromagnetic interaction with the stream of secondary electrons to define a crossed-field interaction region in the space between said cathode and anode, means for applying microwave energy to be amplified to said anode circuit, said cathode electrode structure including a control electrode portion facing said microwave circuit for terminating amplification of the input energy upon termination of the input microwave energy as applied to said microwave circuit, means for insulating said control electrode portion from said secondary emitter portion of said cathode structure, the improvement wherein, the surface of said control electrode which faces said anode circuit is recessed from said anode circuit at the upstream end thereof relative to the downstream end thereof to provide greater anode-to-control electrode spacing at the upstream end of said control electrode than at the downstream end of said control electrode.
 2. The apparatus of claim 1 wherein said control electrode includes an abrupt projection at the downstream end thereof, such projection projecting toward said anode circuit.
 3. The apparatus of claim 1 wherein the anode-to-control electrode spacing is at least 1 1/2 times as great at the upstream end of said control electrode as at the downstream end of said control electrode.
 4. The apparatus of claim 1 including means for applying a bias potential to said control electrode portion which is positive relative to the potential applied to said secondary emitter portion of said cathode structure.
 5. The apparatus of claim 1 wherein said anode and cathode electrode structures are concentrically disposed to define an annular unobstructed reentrant stream crossed-field interaction region therebetween, and said anode including a circuit sever portion interrupting said microwave circuit portion to define an input and an output end of said anode circuit adjacent the ends of said circuit sever.
 6. The apparatus of claim 5 wherein said circuit sever defines a microwave field-free drift region in the space between said sever and said cathode electrode, the surface of said cathode being contoured such that the spacing between said cathode and said circuit sever is greater at the downstream end of said drift region than at the upstream end thereof, and wherein the spacing between the recessed upstream end of said control electrode and said microwave circuit is approximately equal to the cathode-to-anode spacing at the downstream end of said drift region on the axial center plane of the interaction region. 